I'm out looking for a solution for this same issue and came across your post. Sometimes users want a simple report that they can slice and dice to thier own needs. I build reports for our finance team all the time that get turned upside down and inside out depending on what they're looking for. There shouldn't be an issue exporting to Excel. If the report didn't time out when executing, the export to Excel should have been simple.

One of the previous times we ran into this issue there was an Excel file stored in a temp directory that was locked and SSRS wouldn't export another until that file was deleted. Don't know what happened there but it's worth a shot to clean up your temp files. One of our locations has run into this again and it appears this isn't the issue this time so we'll keep looking. Multiple users can run the report and export in seconds but our tech at this site and the user he's helping both have an issue exporting. This tells me it's workstation specific.

My point is, don't give up. There's something wrong if you can't export to Excel. You'll face this again sooner or later with another report. May as well have the know how to fix it when you do. imho