diff --git a/frontend/js/worker.js b/frontend/js/worker.js
index 317b1d8..99f7a58 100644
--- a/frontend/js/worker.js
+++ b/frontend/js/worker.js
@@ -7,12 +7,10 @@ onmessage = async function(e) {
 	let result = [];
 	let res;
 	for (let file of files) {
-		// for communication with exported function 'get_file'
-		self.readFile = file;
 		console.debug(file)
 		
 		try {
-			res = JSON.parse(wasm.run_json_exported("application/x-gzip" == file.type || "application/gzip" == file.type));
+			res = JSON.parse(wasm.run_json_exported(file));
 			// the server depends on the 'name' field to be present
 			res['name'] = file['name']; // retrieve the sample name
 			result.push(res);
diff --git a/frontend/src/extract_comp.rs b/frontend/src/extract_comp.rs
index 1ea57e9..ce90b75 100644
--- a/frontend/src/extract_comp.rs
+++ b/frontend/src/extract_comp.rs
@@ -6,9 +6,9 @@ use crate::{io_utils::get_reader, utils::set_panic_hook};
 
 // Entry points here
 #[wasm_bindgen]
-pub fn run_json_exported(compressed: bool) -> String {
+pub fn run_json_exported(f: web_sys::File) -> String {
     set_panic_hook();
 
-    let fastq_reader = FASTQReader::new(SampleArgs::default(), get_reader(compressed));
+    let fastq_reader = FASTQReader::new(SampleArgs::default(), get_reader(f.into()));
     run_json(fastq_reader)
 }
diff --git a/frontend/src/lib.rs b/frontend/src/lib.rs
index 0d116ed..ca3aaf1 100644
--- a/frontend/src/lib.rs
+++ b/frontend/src/lib.rs
@@ -16,43 +16,60 @@ pub mod io_utils {
     use wasm_bindgen::prelude::*;
     use web_sys::FileReaderSync;
 
-    #[wasm_bindgen(module = "/js/exports.js")]
-    extern "C" {
-        pub fn get_file() -> web_sys::File;
-    }
-
     #[wasm_bindgen]
     extern "C" {
         #[wasm_bindgen(js_namespace = console)]
         pub fn debug(msg: &str);
+
+        #[cfg(debug_assertions)]
+        // get js time
+        #[wasm_bindgen(js_namespace = Date, js_name = now)]
+        pub fn now() -> f64;
     }
-    use std::io::{self, BufRead, Read};
+    use std::io::{self, BufReader, Read};
 
-    // Credit to: mstange on GitHub
-    // See: https://github.com/rustwasm/wasm-bindgen/issues/1079#issuecomment-508577627
     #[derive(Debug)]
-    pub struct WasmMemBuffer {
+    pub struct JSFileReader {
         pos: u64,
+        file_reader: FileReaderSync,
         file: File,
+
+        #[cfg(debug_assertions)]
+        thresh: u64,
+        #[cfg(debug_assertions)]
+        time_start: f64,
     }
 
     #[allow(clippy::new_without_default)]
-    impl WasmMemBuffer {
-        pub fn new() -> WasmMemBuffer {
-            WasmMemBuffer {
-                file: File::from(unsafe { get_file() }),
+    impl JSFileReader {
+        pub fn new(f: File) -> JSFileReader {
+            JSFileReader {
                 pos: 0,
+                file_reader: FileReaderSync::new().unwrap(),
+                file: f,
+                #[cfg(debug_assertions)]
+                time_start: now(),
+                #[cfg(debug_assertions)]
+                thresh: 0,
             }
         }
     }
 
-    impl Read for WasmMemBuffer {
+    impl Read for JSFileReader {
         fn read(&mut self, buf: &mut [u8]) -> io::Result<usize> {
-            let fr = FileReaderSync::new().unwrap();
             let sl = self.file.slice(self.pos, self.pos + buf.len() as u64);
-            let arr = Uint8Array::new(unsafe { &fr.read_as_array_buffer(sl.as_ref()).unwrap() });
+            let arr = Uint8Array::new(&self.file_reader.read_as_array_buffer(sl.as_ref()).unwrap());
             let len = std::cmp::min(buf.len(), arr.length() as usize);
 
+            #[cfg(debug_assertions)] {
+                self.thresh += len as u64;
+                if self.thresh > 100_000_000 {
+                    let new_time = now();
+                    debug(format!("Read {} bytes, {} elapsed", self.thresh, new_time - self.time_start).as_str());
+                    self.thresh = 0;
+                }
+            }
+
             arr.slice(0, len as u32).copy_to(&mut buf[..len]);
 
             self.pos += len as u64;
@@ -62,9 +79,12 @@ pub mod io_utils {
 
     // Reader is a wrapper over BufRead
     // And provides an interface over the actual reading.
-    pub fn get_reader(compressed: bool) -> Box<dyn BufRead> {
+    pub fn get_reader(file: File) -> BufReader<Box<dyn Read>> {
+        let typ = file.raw_mime_type();
+        let compressed = typ == "application/gzip" || typ == "application/x-gzip";
+
         fastq2comp::io_utils::compressed_reader(
-            Box::new(WasmMemBuffer::new()) as Box<dyn Read>,
+            Box::new(JSFileReader::new(file)) as Box<dyn Read>,
             compressed,
         )
     }