Slow and Buggy?

I'm part of a small business and we're trying to use Zoho Sheet to collaborate on spreadsheets that track our client and contractor hours. When we import an Excel spreadsheet into Zoho we run into a bunch of problems:

First, the system seems very slow - taking a minute or more to load the file, move between worksheet tabs, edit cells, etc.

Second, we get some sort of "internal errors" after working within the document for a few minutes.

Third, when we share the file, the person with whom it is shared can see that the originator has the file open, but the orginator cannot see that the person with whom it was shared has it open. Because of this, comments don't pass back and forth.

Fourth, when changes are made on the sheet and saved the updates don't always carry over to those the filed is shared with.

We've also tried the services provided by ThinkFree and ran into some similar issues. Are there configuration issues we're not doing? We have both a PC with Windows XP and a Mac with the latest OS X. Both have similiar problems. We have a high speed Internet connection. Our contractors also have these issues (both with PC and Mac). We see rave reviews of Zoho's services and wonder what we're missing. Any suggestions?

Thanks.