I would say x2 on Thunderbird, whenever possible. Because an E-mail client handles all functions locally, the only data it has to fetch is the E-mail itself. It doesn't have to pull pictures or execute scripts (perhaps pulling large, bandwidth sucking ads) with every action.
Thunderbird is nice for security as well. It has its own key repository, and can stash the mail passwords, all encrypted with a master password.
As for IE... I'd highly recommend going with another browser. I would recommend Chrome, and setting "click to play" on, at the minimum. Since Chrome isolates things quite well, if one browser tab gets infected, it will be far less likely to spread to the entire browser, or the system.
If Chrome isn't loading, or taking a long time, it likely is something misconfigured or some nasties present. I'd probably run both ccleaner and Malwarebytes, one of which is great at cleaning out cruft, the other is good at finding and stomping the bugs.
The only thing is, one can scan for nasties with tons of programs, go through the machine with autoruns (a free utility from Microsoft which scans every task in RAM against 30+ different AV products using VirusTotal), run "sfc /scannow" to check and replace corrupted programs, then use netsh reset to reset the adapter... but there is a point where one is best off backing up all their data, making sure they can reload their applications, and then formatting/reinstalling from scratch.
Oh, don't expect the "reset" feature in W8 and newer to do the job. Tried that recently, and the box completely lost all sound, so ended up just doing a format and reinstall using OS media, which did the job right.