When would one use 临时 versus 及时?
When you use "临时", your point is "wasn't part of the plan".

When you use "及时", your point is "rapidly/promptly".

So if you replacing "好象是临时决定的" with "好象是及时决定的", it also works but the point will become to "They made the decision promptly" from "They made the decision that wasn't the part of the plan (of the meeting)".

Of cource "临时" has other meanings, such as "temporarily", which means a temporary solution which will be replaced by the formal one later on.