Bitlocker is slow

Generally, encryption, and this is regardless of the encryptor used, averages about 100 GB/Hr on traditional hard drives of various types. Most 3rd party encryptors, and BitLocker under Windows Vista and Windows 7, have to encrypt every sector on the drive. A 1 TB 7200 RPM traditional desktop drive encrypts a bit faster than that if it is an internal drive, usually taking about 5-6 hours to encrypt. 5400 RPM 2.5" traditional laptop drives take a bit longer, but internal 1 TB drives still encrypt in 6-8 hours. There is some dependency on processor since the encryption is based on a very complicated mathematical calculation which takes the encryption key that has been generated for the system, and mathematically renders a replacement for the data that each sector contains. Slower processors, especially those in budget notebooks, suffer a significant performance hit usually, and fast Core i7 systems suffer much less. In fact, Core i5 and Core i7 processors actually contain a co-processor circuit specifically designed to handle whole disk encryption, so performance issues of all kinds are going to be less on those CPU's.

SSD's, even under encryptors that have to encrypt the whole volume, encrypt in 1/10 the time of a similar traditional hard drive due to their huge speed difference.

Windows 8.X and Windows 10, at least with BitLocker, break with tradition, and only require that the used space on the drive be encrypted. At least part of the reasons behind the change had everything to do with the size of disks today, and the amount of time it really takes to encrypt them. None of the third party solutions like VeraCrypt, Diskcryptor, BestCrypt, etc. have followed Microsoft's lead in this, at least as of this writing. Since most 1 TB drives in computers are less than 1/5 full when encryption is initiated, under the newer operating systems, encryption, even on traditional hard drives, usually takes less than 2 hours with BitLocker.

USB is where the biggest slowdown occurs with encryption. If you can use USB 3.0, encryption times on a 1 TB volume are maybe only an hour longer than they are for a 1 TB traditional drive hooked up to an interior SATA circuit. USB 2.0, on the other hand, is much slower. It takes about 24 hours per TB to encrypt large traditional hard drives hooked to a USB 2.0 port if every sector of the drive must be encrypted. USB 2.0 has a theoretical maximum throughput of only 480 Mbps, and in actual use, it is hard to get more than 380 Mbps sustained throughput. Compare that to the 6000 Mbps throughput of modern SATA circuits, and it becomes clear why the speed on USB 2.0 is so lackluster.

So, the answer to the posed question is that the encryption times you are seeing are normal, expected, and necessary due to the very nature of whole disk encryption.