Are there any mistakes in the following posting? Especially, I'm not sure about the colored parts.

When a webserver doesn't answer or answers only slowly, all other Opera tabs are blocked too.

The title window says title of the webpage " - Opera (Keine Rückmeldung)", I don't know the verbatim message in English.

This bug is really annoying, besides, Opera is really the fastest browser!

It would be nice if the developers could solve this issue, it only appears in most recent opera versions.
Your posting appears all right.

The 'only' functions in the sentence as an adverb modifying the verb 'answers'.

Yes, the 'besides' is a preposition showing the relation between the two clauses with the latter being the noun clause as its object.
thank you!