slice: Refactor for improved performance.

lib/index.js

@@ -203,12 +203,20 @@ const Channel = function(bufferLength = 0) {
     slice: (start, end = Infinity) => {
       const output = Channel();
       (async () => {
-        for (let index = 0; index < end; index++) {
+        for (let index = 0; index < start; index++) {
           const value = await readOnly.shift();
 
           if (value === undefined) {
             break;
-          } else if (index >= start) {
+          }
+        }
+
+        for (let index = start; index < end; index++) {
+          const value = await readOnly.shift();
+
+          if (value === undefined) {
+            break;
+          } else {
             await output.push(value);
           }
         }