KIS2015 has most improvements though to be honest since 2013 version, they have always worked hard to improve things. 2015 has a lot of work put on preformance. Only thing is the boot time, which exterminator20 already mentioned.
Note that CPU usage, I/O during scan is not as bad anymore. Their only problem which never changed from god knows how many versions is the incompatibility software installed, granted you can still bypass it at your own risk. I did test KAV along with Webroot for a short 2 months give or take and did not find any fault in it, despite Kaspersky yelling at me Webroot was incompatible software with Kaspersy upon installation...