Adding a new slave slowing down previous replicas?
Your new slave server can possibly be slow in some respect and not able to process the queries in the binary log as quickly as the other servers. 

You should also look at the possibility that you may have run a very heavy query that caused the new slave to lag as well as the other slaves.

Regarding the new slave, you may have a performance problem that needs to be resolved. Possibilities could include:


Slow hard disks (you mentioned you are buying new ones)
Bad my.cnf configuration. Flushes too often to disk? flush method isn't O_DIRECT?
Long running queries from master server - consider using row-based replication if you feel it helps your overall system performance.
Caches on the new slave aren't warm for running the queries from the master - do you use INSERT INTO ... SELECT .. FROM statements? or non-deterministic insert statements like ones with sub-queries?
Raid card with battery - do the other servers have it and the slave doesn't?