It depends on your CPU and HDD.
If you have a decent HDD, say a HD103SJ, you should be able to record 1080p footage without lagging provided that you have a capable CPU.
Exceptions are games that stream during gameplay where a second HDD will help a lot.
And as Oldname pointed out, Dolphin doesn't request a lot of read and writes to the HDD, so depending on your HDD, it might or might not be part of the issue.
I think this is more of a CPU issue. Your mileage might vary across different games.
What game are you trying to record?
If you have a decent HDD, say a HD103SJ, you should be able to record 1080p footage without lagging provided that you have a capable CPU.
Exceptions are games that stream during gameplay where a second HDD will help a lot.
And as Oldname pointed out, Dolphin doesn't request a lot of read and writes to the HDD, so depending on your HDD, it might or might not be part of the issue.
I think this is more of a CPU issue. Your mileage might vary across different games.
What game are you trying to record?