[short answer]

Please update Iceweasel to the version 55. It improves the performance in all aspects.

[long answer]
Firefox source code has been updated to the version 55. The highlighted features are:

1.- Browser application UI has been improved to be less intrusive than previous versions.

2.- URL adress resolving now finds the HTTPS adresses when it's avariable.

3.- Enabled the option of modification of number of concurrent processes for faster page loading (ideal for multithreading processors).

4.- Enabled the hardware acceleration mode.

5.- Added webpage screenshot tool.

6.- Added Belarusian (BE) locale.

7.- Many security fixes (https://www.mozilla.org/en-US/security/known-vulnerabilities/firefox/#firefox55).

For more information, go to the following link: https://www.mozilla.org/en-US/firefox/55.0/releasenotes/