When I sync my wallet to 2015 (first half), the speed becomes slower.
Shouldn't it be the other way around? technically synching the first half should be faster, the bottleneck is always processing the blocks, and blocks in the early days were pretty small and the number of transactions was way less than today.
I also want to know the method to connect more nodes because I thought more nodes can help me to sync my wallet faster.
You can't connect to more than 8 nodes, although apparently you could change that rule in the code and recompile it, but that won't really solve your problem and people shouldn't do it, the number of nodes isn't really important, it's the type of nodes you are connected to that matters, but what matters the most is probably your hardware resources, you could increase dbcache size depending on your ram, make sure you leave enough ram for the OS and other software running if you need to use your PC while syncing you would want to reduce that value, below are a few more things you could do to speed-up the process.
Adjusting parameters
There are some parameters that you can look at and consider depending on your system. Always make a backup of your wallet before making any changes. The parameters can be entered at the command line or generally in your bitcoin.conf file, which you would need to exit and restart Bitcoin Core in that case to see the effect. Some things to consider that could make a difference:
I am presuming here that you are running bitcoin-qt which is the GUI version and not bitcoind.
If parameters are used in the bitcoin.conf file, omit the leading -.
-datadir= You could have your datadir on a fast drive. If you move your whole .bitcoin folder you will need to specify this parameter on the command line. Note that if you change your datadir and do not copy the .bitcoin folder over to the new location first, it will create a new wallet and begin downloading the blockchain again from the start.
-dbcache= The default for this is 450. If you have 8GB of RAM, you could easily set this to 4096
par= The default for this setting is auto, but I do not know what the automatic setting does. You could try setting this to -1 which leaves one core free.
-banscore= You could try setting this to 10 to kick off misbehaving nodes sooner.
-listen= You could temporarily set this to 0 to disable incoming connections while you get the initial sync done. Your busy node does not need the additional work.
After the initial sync, you can revert these settings to their defaults but probably leave your datadir wherever you put it.
Some of these parameters are available as options in the bitcoin-qt Settings > Options menu.
Source:
https://bitcoin.stackexchange.com/questions/58369/sync-with-bitcoin-qt-very-slow-0-01