If performance is suffering it may be worthwhile to increase the cache size setting. LR stores previews on the hard drive and spools them on and off as required to avoid the need to constantly recreate them.
It is also possible to alter the amount of RAM that LR is allowed to use via another setting. When I had a 4GB machine, I found it very helpful to increase the allowable amount from the default. With the current 8GB it hasn't been a problem.
If performance is still suffering it may be time to split the catalogue to reduce the amount of data it is running. I store my photos by year_month_date. With the sheer number of photos involved I have found it necessary to catalogue by year. This is also of benefit when backing up the photos, as the catalogue for each year is placed in the folder for that year so it won't be forgotten.