Although this is apparent your question belongs in the Hardware forum.....you ARE handicapped by low system memory AND possibly the use of USB3 external drives.......MANY of which do NOT live up the speeds they are supposed to provide, although SOME do !!!
32GB system memory is the current recommendation for the latest versions of PPro CC 2014....there have been changes made which allow the greater use of system memory to speed things up...I see this in my laptop, which has 24GB system memory....certain PPro activity raises memory use up to 20GB !!
I am currently using the PNY TURBO thumbdrive USB 3 stick and it works great! Though not as fast as an internal SSD, it gives 190MB/sec read speed and almost 100MB/ sec write speed. MANY USB3 devices are not that fast, ( see the hardware forum post on this question).
If you have an NVidia GPU....it it properly ENABLED to provide "GPU acceleration" ? Current versions of PPro allow you to select "checkboxes" inside warning pop up dialogue boxes that allow you to enable " unapproved" Nvidia GPUs.....OLDER versions required the use of a well-known "hack" to enable the card. If you have ATI for your GPU....well....you are SOL....it will not perform anywhere near the NVidia GPU acceleration.
Do you have more than one system hard drive ??? Only one HDD in your system would be a severe handicap, AND would be below the minimum system requirements for PPro. I do not know much about Apple machines,however if you have "thunderbolt" external ports....they would provide a MUCH FASTER CONNECTION for connecting external drives. It is BEST if you have a quality SATA III SSD for your OS drive...not sure what you have.
More info is needed on your system hardware info to give a better answer
Hi! Thanks for your quick response! I do only have one HDD, I believe, since I'm on an iMac. I have the fusion drive, which is part HDD and part flash memory. I do have GPU acceleration enabled, and I am on a supported card. Also, I was mistaken. I am running on a 3.5GHz Quad-Core intel i7 processor.
I was running Premiere Pro on this machine last week, before I upgraded to Yosemite and it was lightning fast - and running from the same external hard drive. I will, however, purchase some extra RAM. I'll look into the PNY Turbo stick too!
Also, my older machine still runs my sequences, off of the same hard drive, with no issues. The main thing I'm wondering is whether Yosemite caused this issue, or if I have something else going on? I can't think of any other reason why an older machine, with the same version of PP would run so much better, especially since playback is not an issue - even with effects applied, such as Denoiser II. Any more thoughts?
Thanks again for your help and sorry for posting in the wrong forum!
I am throwing out an update to this post for anyone else who may have this issue. I reverted my system back to Mavericks (10.9) and the problem has vanished. Premiere Pro is back to running very fast, even when switching between sequences. I'm not sure why this was happening or if anyone else has noticed this issue, but for me, this was the solve.
Hope this helps!