A Class10 is what any high MP camera ( not camera phone) uses (usually) without issue, if you have lagginess with the video camera something else is going on.
I'd try another SD card, it might be the card not the Classification. A Class 6 should be enough but many insist on using a Class 10 just cover all the bases.
My GF has no issues with a Class 6 on her 1520. Nokia in instructional videos showing how to insert an SD Card use a Class 4 and in Tweets to NokiaCare suggest using at least a Class 4 but don't say at minimum use a Class 10. You'll get lots of augments here about that but as I said my GF uses a Transcend 32GB SDHC Class 6 with no issues whatsoever.
Try a different card first if you have one.
Here's what Nokia says about recording video
"Record video to phone memory, not your microSD card
In the same fashion that external storage on your PC is slower and less reliable than your onboard hard drive, memory cards can vary in speeds and reliability. Photographers often run into this challenge when trying to shoot high quality HD video on DSLR cameras. Sometimes the memory card just can?t keep up.
....If you?d rather record the video to microSD, it?s important to use a high-quality brand name memory card."
They say nothing about "at minimum" a Class 10.
Thanks for the input. I don't have a 1520 yet so I have yet to experience and issues but in doing some research for a SD card I came across folks posting the issues I mentioned using class 10 (both were using SanDisk like crav4speed mentioned).
Class 2 and 4 have been mentioned by both MS and Nokia for the 1520, like you mentioned but nothing further than that.
Per Wikipedia:
Secure Digital - Wikipedia, the free encyclopedia
SDHC Speed Class 2.svg Class 2 2 MB/s SD video recording
SDHC Speed Class 4.svg Class 4 4 MB/s High-definition video (HD) recording including Full HD (from 720p to 1080p/1080i)
SDHC Speed Class 6.svg Class 6 6 MB/s
SDHC Speed Class 10.svg Class 10 10 MB/s Full HD (1080p) video recording and consecutive recording of HD stills (high-speed data bus)
UHS Class 1.png UHS Class 1 (U1) 10 MB/s Real-time broadcasts and large HD video files (UHS bus)
UHS Class 3.png UHS Class 3 (U3) 30 MB/s 4K video files (UHS bus)
The reason everyone opts to go with class 10 cards is the transfer speed is 10MB/s so if you are going to use "external storage" you'd want to go with the fastest transfer speeds, i.e. USB 2.0 vs USB 3.0. Back when I had my HTC Evo 4G, I initially had a class 2 card and had some serious slowness in the camera app (default was save to SD). I then replaced it with a class 10 card and noticed the camera app running faster for both video and stills. Nokia also doesn't tell us what the bus connection speed for the SD card slot is so if its at say 6MB/s a class 10 card really wont give you any benefit as opposed to a class 6.
Also your statement about class 10 being reserved for high MP digital cameras and not smartphone cameras, I really disagree. The Lumia 1520 and 1920 are 20MP and 40MP so they really don't fall into your traditional camera phone. Class 6 should work well, which is one reason why your GF's 1520 hasn't had any issue.
Lastly I find it funny that Nokia says: "it?s important to use a high-quality brand name memory card" yet the issues have been reported with SanDisk which is considered just that. Personally I have never cared for SanDisk products, they are overpriced and you can get better performance

rice in other brands.